The invention relates generally to traction and anti-skid braking vehicle wheel control systems. More specifically, the invention concerns a method for obtaining an estimate of vehicle reference speed and acceleration suitable for use in determining wheel slip in such systems.
Known anti-skid braking control systems use various mathematical manipulations to estimate the speed of a vehicle for comparison with the rotational speed of a given vehicle wheel to calculate wheel "slip" for that wheel, wheel slip being a parameter needed in order to make decisions concerning wheel lock control.
Due to the interaction between the wheels and the road surface, especially during braking, the actual reference speed of the vehicle itself with respect to the ground is usually different from the speed of a particular wheel. Hence to obtain accurate estimates of vehicle speed, conventional anti-skid braking systems or traction control systems have employed various techniques to simulate a "fifth wheel" which accurately tracks the speed of the vehicle itself. Such known mathematical manipulations typically require averaging or comparison with other wheel speeds to obtain an estimate of the wheel slip at a given single wheel.
Additionally, known systems will not function properly with a non-standard size tire mounted to one of the wheels. This situation is encountered, for example, where a miniaturized spare is being temporarily used after the occurrence of a flat tire. Also, it has been conventionally necessary with prior anti-skid braking systems to disengage the 4-wheel drive option of 4-wheel drive vehicles during anti-skid braking controlled stops in order to obtain a valid reference speed in accordance with the selected mathematical manipulation.
The typical known approaches to mathematically estimating reference speed use averaging procedures and deterministically limit the allowed wheel acceleration to obtain a reference speed from which wheel slip can be estimated or calculated. In effect, such prior approaches typically require some wheel (usually the rear wheels) to be braked at less than maximum torque so that they may be used to estimate the vehicle speed--i.e. the rear wheels are in effect braked at a decreased amount such that they may simulate a "fifth wheel" of the vehicle.
With the advent of mid-engine cars and high performance vehicles, it may no longer be possible to proportionally control the hydraulic system feeding the rear brakes to maintain low slip at the rear wheels such that they may be used for reference vehicle speed estimates and still maintain the high performance of the newer vehicles currently being introduced.
Known approaches to estimating vehicle reference speed are disclosed in the following U.S. Pat. Nos.: 3,934,938-Kuwana et al., 3,953,080-Bremer, 3,964,796-Bremer, 3,985,396-Kuwana et al., 4,545,623-Sato et al., 4,648,663-Nomura et al., 4,656,588-Kubo, 4,665,490-Masaki et al.